1. Field of the Invention
This invention relates to external preparations containing, as active ingredients, kojic acid or derivatives thereof which are free of discoloration with the passage of time.
2. Prior Art
Kojic acid and derivatives thereof are known to show inhibitory action against melanin which tends to be formed in the human skin. Also known are various liniments containing these substances as active ingredients. These include whitening cosmetics comprising kojic acid as an active ingredient (Japanese Patent Kokai No. 53-3538 and Japanese Patent Publication No. 56-18569), whitening cosmetics comprising, as active ingredients, a mono- or di-ester of kojic acid with an aliphatic carboxylic acid (Japanese Patent Kokai No. 56-7710 and Japanese Patent Publication No. 60-9722), whitening cosmetics comprising a kojic acid ester with an aromatic carboxylic acid, such as cinnamic and benzonic acids as active ingredients (Japanese Patent Publication No. 60-10005), and ointments to inhibit melanogenesis comprising kojic acid as an active ingredient (Japanese Patent Publication No. 61-10447).
As stated above, kojic acid and derivatives thereof are substances capable of effectively inhibiting the formation of melanin in the human skin. It is known that these substances are effective in making the human skin fair with no harmful effects to the skin when cosmetics and external preparations containing them as main ingredients are applied to human skin.
However, kojic acid or derivatives thereof manufactured as external preparations tend to become discolored with the passage of time during the steps of storage and distribution, which results in lowered value as commodities.